Technique for enhancing the ability of polymer to bond with adhesive and resultant polymer

ABSTRACT

HYDROCARBON, FLUOROCARBON AND POLYAMIDE POLYMERS DESTINED FOR BONDING WITH ADHESIVES ARE MELTED UPON A HIGH ENERGY METAL OR METAL OXIDE SURFACE, COOLED AND SEPARATED THEREFROM BY DISSOLUTION OF THE LATTER. THE RESULTANT MATERIALS ARE CAPABLE OF BONDING WITH ANY CONVENTIONAL ADHESIVE, YIELING SUPERIOR BOND STRENGTHS.

United States Patent US. Cl. 260-949 5 Claims ABSTRACT OF THE DISCLOSUREHydrocarbon, fluorocarbon and polyamide polymers destined for bondingwith adhesives are melted upon a high energy metal or metal oxidesurface, cooled and separated therefrom by dissolution of the latter.The resultant materials are capable of bonding with any conventionaladhesive, yielding superior bond strengths.

This application is a division of copending application, Ser. No.616,785, filed Feb. 17, 1967, US. Pat. 3,520,753.

This invention relates to a technique for enhancing the ability ofpolymeric materials to form a seal with an adhesive and to the resultantpolymers. More particularly, the present invention relates to atechnique for treating hydrocarbon, fluorocarbon or polyamide polymersto improve their ability to form an adhesive bond.

It has generally been accepted in the adhesives industry that the notedpolymers generally cannot be bonded with commercially availableadhesives in the absence of a preliminary surface modification, such asoxidation, defluorination, and so forth, Unfortunately, the varioussurface modifications proposed by the industry tend to improve oneproperty at the expense of another, so resulting in a composition whichis undesirable in many applications.

A technique for overcoming these limitations is described by R. H.Hansen and H. Schonhorn in US. Pat. 3,462,335, issued Aug. 19, 1969. Thetechnique described therein involves bombarding a hydrocarbon,fluorocarbon or polyamide polymer, destined for bonding with anadhesive, with a stream of an excited inert gas. The resultantcompositions were found to retain their original electricalcharacteristics as well as the chemical integrity. Although thistechnique has proven completely satisfactory in all applications, theattention of workers in the art has been focused upon more economicalprocedures for effetcing similar ends.

In accordance with the present invention, such end is attained by anovel procedure wherein the polymer destined for bonding is melted upona high energy surface so as to effect wetting thereof, cooled andseparated from the surface by dissolution of the latter in a suitablesolvent. Compositions treated in the described manner are also found toretain their original electrical characteristics and chemical integrityand are capable of bonding with conventional adhesives so as to resultin structures evidencing tensile shear strengths comparable to thoseobtained by means of the technique described in the copendingapplication alluded to hereinabove.

It will be understood by those skilled in the art that the main impactof the present invention resides in the discovery that the notedpolymers may be economically bonded with conventional adhesives withoutundergoing a preliminary surface modification with the concomitantdegradation of electrical properties.

3,635,938 Patented Jan. 18, 1972 The invention has been describedlargely in terms of bonding polymers with epoxy adhesives. However, itwill be understood that the polymers described herein may be bonded withany conventional adhesive utilized in the industry, for example,polyamides, polyurethanes, polysulfides, silicone, and so forth.

The polymers employed in the practice of the present invention may beselected from among hydrocarbon, fluorocarbon, or polyamide polymersevidencing weak boundary layers as manifested by their inability to formstrong adhesive joints. Typical polymers suitable in the practice of thepresent invention are polyethylene, polypropylene, nylon,polytetrafluoroethylene, poly(vinyl fluoride), and so forth.

The adhesive employed herein may be selected from among any of thecommercially available materials. However, in order to obtain high jointstrengths, the adhesive is required to wet the substrate appreciably, idest, it should evidence a surface tension less than 50 dynes per cm. Inorder to utilize material manifesting surface tensions beyond the notedmaximum, a surfactant may be employed to attain the required level. Aparticularly useful adhesive for the practice of the invention comprisesa mixture of the diglycidyl ether of bisphenol A anddiethylaminopropylamine.

As noted, the inventive technique involves melting a polymeric materialonto a high energy surface, which, for present purposes, is defined asany surface manifesting a surface tension of at least 50 dynes per cm.Materials particularly suited for this purpose are gold, aluminum,mercury, tin, iron, and so forth.

The first step of the inventive technique involves cleansing both thepolymer of interest and the high energy surface with conventionalsolvents for the purpose of removing surface debris. Thereafter, thepolymer is heated for a time period and at a temperature suflicient toeffect melting thereof, the specific temperature and duration of heatingbeing dependent upon the polymer selected. It Will be understood bythose skilled in the art that the ultimate joint strength of the bond isdependent in part upon the extent of Wetting attained in this stage ofthe operation, an optimum condition being the attainment of a contactangle of 0. Accordingly, the duration of heating is dictated byconsiderations relating to the time required to obtain a contact angleapproaching 0.

Subsequent to melting, the resultant assembly is cooled to roomtemperature by any suitable means. It has been theorized that during thecooling cycle of the process extensive nucleation of the polymer occurs,resulting in the formation of a transcrystalline region which in turncauses the formation of a plurality of entanglements in the surfaceregion which act as crosslinks and preclude the formation of a weakboundary layer.

Thereafter, the cooled polymer is separated from the high energy surfaceor substrate by dissolving the surface in any suitable solvent. Studieshave revealed that removal by other techniques as, for example, peeling,results in the destruction of the surface region and may cause exposureof a deleterious weak boundary layer.

Next, the polymer may be stored until ready for use or immediatelybonded with the adhesive of interest. The adhesive is applied uniformlyto the surface of a sheet of the polymer and the coated polymermaintained at a temperature ranging from room temperature to a pointjust below the melting temperature for a period suflicient to permitcuring of the adhesive, thereby forming a structural joint between thepolymer and the adhesive.

Examples of the present invention are described in detail below. Theseexamples are included merely to aid in the understanding of theinvention and variations may be made by one skilled in the art withoutdeparting from the spirit and scope of the invention.

EXAMPLE I A 1" x 3" x mil sample of polyethylene obtained fromcommercial sources was washed with acetone, dried in air and placedbetween a pair of 1 x 3" sections of 0.6 mil aluminum foil previouslytreated with a (sulfuric acid-sodium dichromate) (sulfochromate)solution. The resultant assembly was next placed between a pair ofheated platens and heated at a temperature of 175 C. for 30 minutes.Then the assembly was cooled to room temperature by passing cold waterthrough the platens. The polyethylene was next separated from the foilby dissolving the latter at room temperature in a percent sodiumhydroxide solution. The polyethylene was then examined and it wasdetermined that the surface resistivity and wettability were the same asthe starting material.

An epoxy resin diglycidyl ether of bisphenol-A was selected for use inpreparing an epoxy adhesive. The resin obtained from commercial sourcesevidenced an epoxy equivalent weight of 179, a total chloride contentless than 0.1 percent, by weight, and a viscosity of 6,400 centipoisesC.

Diethylaminopropylamine obtained from commercial sources was distilledunder nitrogen through a 6 inch Vigreaux column, and the first fractiondiscarded. The product distilling at 68 C. and 26 millimeters ofpressure was stored in a dark container prior to use.

An epoxy adhesive was prepared by mixing 100 parts, by weight, of theabove-described resin with 7 parts, by weight, of thediethylaminopropylamine. The mixture was thoroughly stirred untiluniform and applied immediately to the surface of the polyethylene. Theresultant assembly was maintained at a temperature of 70 C. forapproximately 15 hours. In order to determine tensile shear strength ofthe resultant structure, composites were prepared in accordance with thefollowing procedure.

Metal tensile shear adherends 5 x l x inch of 2024 T3 aluminum, obtainedfrom commercial sources, were selected. The surfaces of the adherendswere subjected to vapor degreasing in trichloroethylene in accordancewith conventional procedures and subsequently etched for 7 minutes at 65C. in a solution comprising one part, by weight, sodium dichromate,parts, by weight, water, and 10 parts, by weight, of 95 percent sulfuricacid. Following the etching step, the adherends were rinsed for fiveminutes in running tap water, for one minute in running distilled water,and then dried in a forced air oven at 60 C. The adherends were thenstored in a desiccator until ready for use.

For the measurement of tensile shear strength, composite test piecescomprising aluminum-epoxy adhesivepolyethylene-epoxy adhesive-aluminumwere prepared for bonding in a device designed to maintain a /2"overlap, the thickness of the epoxy adhesive being maintained constantby insertion of a piece of 0.003" diameter wire in each glue linebetween the aluminum and polyethylene. Bonding of the aluminum to theepoxy adhesive in polyethylene structure was effected at a pressure of20 lbs. per square inch by placing the composites in forced air ovensmaintained at 70 C. for 16 hours. The bonded structures were tested inaccordance with ASTM D1002-53T, except that the strain rate was 0.1 perminute. The tensile shear strength of the structure was approximately2500 lbs. per square inch at 23 C.

For comparative purposes, a prior art polyethyleneepoxy adhesive bondwas formed by directly melting polyethylene between aluminum stripscoated with the abovedescribed adhesive. Tensile shear strength of theresult ant structure as determined in accordance with ASTM Dl002-53T wasapproximately 2000 lbs. per square inch at 23 C.

EXAMPLE II The procedure of Example I was repeated with the exceptionthat nylon, obtained from commercial sources, was employed. The tensileshear strength of the resultant structure was approximately 3000 p.s.i.at 23 C.

EXAMPLE III The procedure of Example I was repeated with the exceptionthat polytetrafiuoroethylene, obtained from commercial sources, wasemployed. The tensile shear strength of the resultant structure wasapproximately 2500 p.s.i. at 23 C.

What is claimed is:

1. A polymer coating capable of forming a bond with an adhesive, saidpolymer being selected from the group consisting of (a) hydrocarbons,(b) fiuorocarbons, and (c) polyamides, characterized in that the saidpolymer has been maintained, while melted, in contact with a high energymetal or metal oxide surface evidencing a surface tension of at least 50dynes per centimeter for a time period sufficient to effect wettingthereof, cooling the resultant assembly to room temperature andseparating the surface from the polymer coating by dissolving saidsurface in a solvent therefor.

2. A polymer in accordance with claim 1 comprising polyethylene.

3. A method in accordance with claim 1 wherein said polymer ispolyethylene.

4. A method in accordance with claim 1 wherein said high energy surfaceis aluminum.

5. The method of treating a polymer selected from the group consistingof hydrocarbons, fiuoroearbons and polyamides, to produce on the polymera surface having improved ability to form an adhesive bond comprisingmelting a polymer selected from the group consisting of hydrocarbons,fiuorocarbons and polyamides onto a metal or metal oxide surfaceevidencing a surface tension of at least 50 dynes per centimeter andmaintaining the polymer coating in contact with the surface for a timeperiod sufficient to effect wetting thereof, cooling the resultantassembly to room temperature and separating the surface from the polymercoating by dissolving said surface in a solvent therefor.

References Cited UNITED STATES PATENTS 2,668,134 2/1954 Horton 26094.9X

JOSEPH L. SCHOFER, Primary Examiner W. F. HAMROCK, Assistant ExaminerU.S. Cl. X.R.

